This invention relates to a printing stylus for chart recorders and the like and particularly a thermally activated stylus which records information on a heat sensitive recording medium. Such stylus constructions are often operated by producing heat in one location and thereafter conducting such heat to a more localized area subsequently adapted for contact with the surface of the heat sensitive paper or by producing the heat directly at the localized area in which it is desired. The former type construction has the disadvantage of generating a substantially greater amount of heat than is necessary while the latter involves constructions which may exhibit rather complex physical configurations or semi-conductor tecnology involving rather high initial fixed costs. An example of the former type construction is shown in U.S. Pat. No. 3,689,937. An example of the latter is shown in U.S. Pat. 3,814,897 issued June 4, 1974.
An object of the present invention is the provison of a thermal stylus construction which avoids the prior drawbacks and yet which serves to localize heat produced at the point of use in such a manner so as to diminish power requirements, eliminate the necessity of a heat reservior and to insure fast warm-up.
A further object of the present invention is the provision of a stylus construction of the above-indicated type which further exhibits desirable wear characteristics and which is of straightforward physical construction such that it may be formed at a relatively low cost and involving generally uncomplicated technology.
These and other objects of the present invention are accomplished by the provision of a thermal stylus in which a pair of electrical conductor strips are disposed in contact with opposed surface portions of a generally elongated insulative member having low thermal conductivity and which terminates in a relatively small tip portion. A coating of an electrically resistive film is disposed over the member tip. The coating contacts the spaced lower ends of the strip so as to form a current path therebetween and electrical connections are attached to the upper ends thereof for connecting the path to a power source wherein current passing through the path heats the tip of the stylus. The coating may be of a wear-resistant, ceramic material such as glass with electrically conductive particles dispersed therein such that desirable wear characteristics are produced in a straightforward economical manner.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when in connection with the accompanying illustrative drawing.